Flagella are natural rotary motors that promote swimming motility required by many bacterial pathogens to navigate environments, infect hosts to promote disease, and promote biofilm formation. Compared to peritrichous bacterial pathogens such as E. coli and Salmonella species, many significant pathogens including Campylobacter jejuni, Vibrio cholerae, Helicobacter pylori, and Pseudomonas aeruginosa produce only a very limited number of flagellar motors specifically at poles of a bacterial cell. Furthermore, these polar flagellar motors promote higher torque and greater velocity of motility compared to flagellar motors of peritrichous bacteria. Thus, alternative paradigms must exist to account for how polar flagellar motors form and function in bacterial pathogens. We have explored flagellar biogenesis in C. jejuni to discover new paradigms for how polarly-flagellated bacterial pathogens regulate flagellar gene transcription, spatially and numerically regulate flagellar biogenesis, and alter flagellar motor structure to power flagellar rotation for motility.We also found evidence for general conservation of these mechanisms in a range of polarly-flagellated pathogens. In this proposal, we will continue to use C. jejuni as a model system to understand how the flagellar motor, which is a universal virulence and colonization determinant, forms and functions in a wide range of polarly-flagellated bacterial pathogens. Our work will also continue to reveal how the bacterial flagellum functions beyond motility to influence signal transduction pathways, spatial organization of the bacterial cell, and cell division in C. jejuni. n Aim 1, we will examine how conserved flagellar two-component regulatory systems (TCSs) of polarly-flagellated pathogens detect formation of the flagellar type III secretion system (T3SS) and surrounding ring structures to initiate signal transduction required for flagellar gene transcription. We will also exploit the natural engineering of T3SS formation to activation of the C. jejuni flagellar TCS to determine molecular requirements of proteins to form T3SSs in bacterial pathogens. In Aim 2, we will explore how the conserved FlhG ATPase of polar flagellates and a possible unusual flagellar C ring composition function together to numerically regulate flagellar biogenesis and spatially control cell division in C. jejuni. In Aim 3, we will exploit mutagenesis and electron cryotomography technologies to decipher the ordered biosynthetic pathway for formation of flagellar disk appendages, which are required for powering flagellar rotation and creation of a functional polar flagellar motor. Accomplishment of these aims will promote new insights for how: 1) TCSs detect and perceive stimuli for initiation of signal transduction required for flagellar gene expression; 2) conserved proteins initiate T3SS formation in bacterial pathogens; 3) bacteria numerically regulate polar flagellar motor biogenesis; 4) flagellar proteins are used in alternate biological activities such as signal transduction, cell division, and spatial organization of the cell; 5) polarly-flagellated pathogens employ disk appendages to diversify flagellar structure and power flagellar rotation to generate torque resulting in high velocities of motility.